The murine immune response to heterologous insulins is qualitatively controlled by H-2-linked immune response genes. Previous data has demonstrated that nonresponder mice are deficient in helper T (Th) cell activity. However, this deficiency does not result from an absence of specific Th cells, since helper function can be demonstrated in nonresponders after depletion of dominant suppressor T (Ts) cells. These latent Th cell subpopulations are cross-reactive for mouse (self) insulin. Based on these observations, we submit the hypothesis that MHC-linked unresponsiveness results from mechanisms invoked to maintain immunological self tolerance. We predict that, in nonresponders, foreign and self insulins are cross-reactive at the level of the T cell receptor. To test this hypothesis, we propose an investigation of the precise ways in which regulatory T cells recognize foreign and self insulin, in nonresponders as compared to responders. We plan to establish T cell hybridomas representing these regulatory subpopulations and to characterize their phenotype, function, activation requirements, and antigenic epitope specificities. The availability of defined insulin-derived peptides should aid in this endeavor. These findings will then be tested using heterogeneous T cell populations both in vitro and in vivo. In addition, we propose a comparative analysis of the frequencies and priming requirements of insulin-specific Th cells from responders and nonresponders. Through these studies, we expect to gain new insight into the extent and nature of antigenic recognition by regulatory T cells in the circumstance of MHC-linked unresponsiveness and self tolerance.